At present multiple pass optical systems featuring a long optical path length are an integral part of every IR spectrophotometer for examination of low-concentration gases or those having very faint absorption bands. Such systems are applicable both for qualitative and quantitative analyses.
One prior-art multiple pass optical system is known (cf. Journal of the Optical Society of America, 1940, v. 30, 338-342, H. D. Smith, L. R. Marshall), wherein the radiant flux emitted by the illuminator passes through the entrance aperture of the casing onto a mirror objective rotatably mounted across the path of the radiant flux, and further onto the unit of intermediate images of the illuminator radiation source, from which the radiant flux is directed again onto the reflecting objective and, on being reflected from said objective, the radiant flux completes the multiple pass cycle and emerges from the system through the exit aperture of the casing. In the above described optical system, the unit of intermediate images of the illuminator radiation source comprises two plane mirrors arranged at an angle to each other.
However, when focusing the intermediate images of the illuminator radiation source in the aforesaid system, the marginal beams fail to get restricted on the flat mirror surfaces, whereby when reflections of the beam are multiply repeated a considerable proportion of the emitted radiant flux is scattered beyond the faces of the reflecting objective (that is, vignetting of the slanted beams occurs).